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Intracellular phosphate ( P) is normally maintained at a fairly constant concentration in Escherichia coli, mainly by P transport systems and by the 'phosphate balance' between P and polyphosphate (polyP). We have reported previously that excess uptake of P in a phoU mutant results in elevated levels of polyP. Here, we found that the elevated levels of polyP in the mutant could be reduced by the overproduction of YjbB, whose N-terminal half contains Na/ P cotransporter domains. The rate of P export increased when the YjbB overproducer grew on a medium containing glycerol-3-phosphate. These results strongly suggested that YjbB reduced the elevated levels of polyP in the phoU mutant by exporting intracellular excess P. [ABSTRACT FROM AUTHOR]